It's not any different than telling an automated phone voice tree system that you plan on killing someone and then being surprised that your words were later heard by a human. She absolutely told a company's computer. She sent the message.
The law may have been intended for more direct threats to a person as a means of intimidation, but that's a separate conversation.
I'm not really sure that this can be likened to a diary when it is called a "chat" but that's for the legal system to determine, not me sitting on my couch.
And yes, some laws are "strict liability", I don't think this one is.
She wouldn't have intended her housemate to open it, so it wasn't a threat.
And her housemate didn't intend to open it, so it wasn't Obstruction of Correspondence. https://www.law.cornell.edu/uscode/text/18/1702